NSSDC Archives Newly-Digitized Alouette/ISIS Data

Volume 12, Number 2, June 1996 By Dieter Bilitza

By early July 1996 the first data from the Alouette/International Satellite for Ionosphere Studies [ISIS] Topside Ionospheric Sounder Data Project are expected to flow into NSSDC's near-line mass storage system (the NASA Data Archive and Distribution Service [NDADS]). Soon afterwards the data will flow at a rate of about 1.5 Gbytes per day. The topside sounder ionograms are being digitized from their original 7-track analog telemetry tapes by a Goddard Space Flight Center (GSFC) team led by Dr. R. F. Benson (Laboratory for Extraterrestrial Physics). This important data set was in danger of being lost permanently when the Canadian Public Archives were no longer able to store the more than 100,000 Alouette/ISIS telemetry tapes. A "Mayday, Mayday" alert of H. G. James (Communication Research Laboratory, Ottawa) concerning the imminent loss of these tapes was picked up by the relevant Space Physics Data System Data Evaluation Panel, and rescue of this data set was assigned highest priority. As a result of these efforts and recommendations, the Alouette/ISIS project at GSFC was initiated. NSSDC maintains World Wide Web (WWW) pages with information about the Alouette/ISIS project at
http://nssdc.gsfc.nasa.gov/space/isis-status.html.

Robert Benson (right) and Dieter Bilitza compare newly-digitized ISIS ionograms with the early film version.

The Canadian Alouette 1 and 2 and ISIS 1 and 2 satellites were launched into polar orbits in 1962, 1965, 1969, and 1971, respectively, and each collected data for ten or more years, covering altogether close to two solar cycles. These satellites carried a number of in situ experiments, and most importantly, they were the first satellites to carry ionosondes into orbit and to provide a full mapping of the topside ionosphere from satellite altitude down to the point of highest density (the F-peak). The highly successful program was ahead of its time in many ways. Unfortunately, it was also ahead of its time in terms of the computer power required to analyze the huge volume of ionograms recorded at the more than 70 ground stations worldwide. Typically, ionograms were taken every 24 seconds for about 15 minutes per pass over any given ground station. Only about 15% of the telemetry tapes have been processed to ionograms, and an even lower percentage was converted into electron density profiles. The science yield from this "small" data portion is impressive (nearly 1,000 scientific publications), and to this day this small portion has been the defining standard data set for the topside ionosphere; for example, the data were used to establish the Bent and International Reference Ionosphere topside models.

With the current processing rate estimate of 15 tapes per day, the project team would be able to save the contents of about 10,000 tapes assuming a two-year project duration. With help from NSSDC and input from the community, Dr. Benson established a selection of more than 13,000 tapes providing a good cross-section in terms of global and temporal coverage and also including time periods of special community interest. These tapes were transported on two trucks from Canada and arrived at GSFC in early March 1996.

The ionograms are digitized at the rate of 40,000 samples/second providing a measurement every 25 microseconds corresponding to an apparent range interval of 3.75 km. The data will be automatically sent by FTPs to NSSDC and automatically ingested onto NDADS. In addition to the full-resolution ionograms (about 1Mbyte/ionogram) NSSDC will also receive (and ingest onto NDADS) ionogram files for which the apparent range resolution has been reduced to 15 km by averaging over four consecutive 25-microsecond apparent range (about 250 Kbyte/ionogram). It is planned to make the averaged ionograms also available in Common Data Format (CDF) for use with NSSDC/Space Physics Data Facility's (SPDF) CDF-related display/plot software systems (CDAWeb, Interactive Data Language [IDL] tools). In addition, NSSDC will receive a header file for each satellite pass with orbit information and also the pulse code modulation (PCM) data file containing header, time, sounder receiver automatic gain-control voltage (AGC), frequency-marker information, and data from all other instruments. The current projection is a processing rate of 15 tapes per day. With typically 80 ionograms per tape, the data inflow to NSSDC will be about 1.5 GBytes per day.